As a lowly ex EMT2 (now paramedic) below 3,000 ft altitude a patient with blood O2 in the 80's gets supplemental O2 and a free ride to the ER. Below 88 things get serious fast. If you see low readings get to a hospital - fast!
Adjustments for altitude compensation need to be done by a qualified M.D. Folks with apnea may with a dictor's endorsement combine therapy with an oxygen concentrator. This is all M.D. stuff but it may allow a borderline patient more flexibility as far as OSA & CSA and altitude restrictions. I NEVER rely on an M.D.'s advice alone. Sleep therapists can offer a treasure trove of info that when counter-checked with an MD often results in a "Oh yeah. That's right". But do not violate the MD's Rx.